Because bullet spin slows down VERY slowly, while velocity slows down pretty rapidly.
A bullet started at 2000 fps from a 1-12 twist is spinning at 120,000 rpm when it hits something near to the muzzle, say some test media meant to see how much the bullet will expand downrange.
A bullet started at 3000 fps from a 1-12 twist starts at 180,000 rpm, and is still spinning almost that fast when it hits something out where velocity has dropped to 2000 fps.
